1. Field of the Invention
The present invention relates generally to a multicarrier communication system using multiple antennas, and in particular, to an apparatus and method for calibrating transmission paths.
2. Description of the Related Art
A smart antenna system and a Multi-Input Multi-Output (MIMO) system are multiple antenna systems. The smart antenna system is an adaptive antenna array system using multiple antennas to automatically optimize a radiation pattern and/or a reception pattern according to a signal environment. The smart antenna system receives only a signal from the direction of an intended user and significantly reduces the level of noise caused by multiple access interference from the other directions by controlling the gain and phase of a signal in each antenna in a Base Station (BS). As frequency utilization reaches its limit, active studies have recently been conducted on improving the quality of mobile communications and systems suitable for high-speed data transmission. As a result, the smart antenna system is attracting more and more interest.
When the BS sends a signal omni-directionally to all Mobile Stations (MSs) within its coverage area, each of the MSs receives interference from signals for the other MSs as well as its own signal, thereby decreasing Signal-to-Noise Ratio (SNR). In contrast, the BS can steer a signal only in the direction of an intended MS by beamforming using the smart antenna system. Therefore, power is saved for signal transmission and interference is also reduced. Even within the same coverage area of the BS, an intended MS is actively located and a signal is directionally sent/received to/from the MS, minimizing interference to the other MSs. As a consequence, the BS may allocate saved power to other MSs and the decrease of inter-neighbor cell interference increases the channel capacity of the BS.
In applying the smart antenna system to Orthogonal Frequency Division Multiple Access (OFDMA) using multiple orthogonal frequency carriers, a beam weight vector is applied to the subcarrier of each antenna during beamforming in order to apply directivity to an intended direction. The beamforming takes place in a digital part and the resulting beams must be provided to the physical antennas prior to radiation in the air. However, the phases and amplitudes of the beam signals are distorted by an amplifier, an upconverter/downconverter, and a cable which have non-linear characteristics in the BS.
Accordingly, the smart antenna technology must be implemented alongside calibration technology to compensate for the phase and amplitude distortions. The overall performance of the smart antenna technology is predominantly dependent on calibration accuracy. In other words, accurate calibration improves the performance of the smart antenna technology by minimization of amplitude and phase mismatches. The calibration technology applies commonly to the downlink from the BS to the MS and the uplink from the MS to the BS.
FIG. 1 is a block diagram of a typical transmission path calibrating apparatus in a multicarrier communication system using multiple antennas.
Referring to FIG. 1, a calibration processor 101 generates a reference signal to be propagated in a particular transmission path. A baseband processor (not shown) provides the reference signal to a non-linear system 102 in the transmission path. The non-linear system 102 oversamples the reference signal, modulates the oversampled signal to a Radio Frequency (RF) signal, and sends the RF signal in the transmission path. An RF coupler/combiner 103 couples the RF signal and transfers the coupled reference signal in a reception path 104. The calibration processor 101 estimates the change of phase and amplitude in the reference signal using the reference signal and the coupled reference signal and calculates a calibration vector based on the estimates. The calibration takes place sequentially for transmission paths.
FIG. 2 illustrates a conventional calibration method. In OFDMA, phase and amplitude changes occurring in the non-linear system must be found for each subcarrier. Hence, a reference signal is sent over a total frequency band and a calibration vector for each transmission path is estimated using the response of the reference signal.
Referring to FIG. 2, the BS generates a reference signal 201 over a total frequency band, for calibration. The reference signal 201 is sent on subcarriers 205 through an Inverse Fast Fourier Transformer (IFFT) 203, travels in a predetermined path 207, and then is demodulated in a calibration processor Fast Fourier Transformer (FFT) 209. The BS estimates phase and amplitude differences for each transmission path in which the reference signal has been propagated in the above procedure.
Since calibration is performed sequentially for the transmission paths, calibration time is increased in proportion to the number of the transmission paths. The transmission characteristics of each path vary with time, which implies that calibration accuracy may be decreased. Also, data throughput is reduced because the reference signal is sent over the total frequency band in a predetermined transmission path during calibration of each transmission path.